1. Technical Field
The present invention relates to the field of devices which use non-volatility of ferroelectric material. More particularly, it relates to a configuration of a functional block which acts as an element of device configuration to transfer polarization when using polarization of ferroelectric material as memory elements.
2. Related Art
Recently, in the field of memories, non-volatile memories which are electrically writable and erasable have been growing in importance. There are various types of non-volatile memory, but ferroelectric memories have been receiving attention because of their high speed, low voltage characteristics, and low power consumption.
For example, as shown in FIG. 43, a ferroelectric thin film 4340 is placed between an electrode 4341 and electrode 4342 to form a ferroelectric capacitor 4349, which is used as an element of a memory cell. Besides, there is a so-called 1T1C (1-transistor, 1-capacitor) ferroelectric memory, in which a memory cell consists of an insulated-gate field effect transistor 4412 and ferroelectric capacitor 4411 and the groups of the memory cells are arranged in a matrix on word lines 4413, bit lines 4414, and plate lines 4415 as shown in FIG. 44.
FIG. 42 is a characteristic curve showing a relationship between applied voltage of the ferroelectric capacitor 4349 indicated by a broken line in FIG. 43 or the ferroelectric thin film 4340 and internal polarization. In FIG. 42, when an applied voltage V is applied, reverse polarity polarization is induced in the ferroelectric thin film. This state corresponds to characteristic point 4204. Subsequently, even if the applied voltage is reduced to 0, residual polarization remains in the ferroelectric thin film, resulting in characteristic point 4205. Then, when a voltage is applied in the positive direction, the residual polarization disappears, resulting in characteristic point 4206. Then, when the applied voltage is changed to −V, reverse polarity polarization is induced, resulting in characteristic point 4201. Then, even if the applied voltage becomes 0, residual polarization remains in the ferroelectric thin film, resulting in characteristic point 4202. Then, when a voltage is applied in the positive direction, the residual polarization disappears, resulting in characteristic point 4203. Then, when a positive voltage V is applied, the characteristic curve returns to characteristic point 4204. Thus, as can be seen from the characteristic curve in FIG. 42, ferroelectric material presents hysteresis characteristics depending on the applied direction and history of the applied voltage. Also, the polarization induced by the application of a voltage is retained as residual polarization even if the applied voltage is reduced to 0. The residual polarization does not disappear even if a voltage is applied in the reverse direction, provided that the voltage does not exceed coercive voltage. The above-mentioned hysteresis characteristics and residual polarization feature of ferroelectric material are used for non-volatile memories.
FIG. 45 is a sectional view showing a structure of a ferroelectric transistor which is formed as a field effect transistor consisting of a gate electrode 4501, source electrode 4502, drain electrode 4503, and bulk or channel 4509, and which is provided with a ferroelectric thin film 4500 formed directly underneath the gate electrode 4501. In the ferroelectric transistor in FIG. 45, a threshold voltage of the field effect transistor changes with the polarity and magnitude of the residual polarization of the ferroelectric thin film 4500, causing a source-drain current to change.
Also, there is a ferroelectric memory which makes use of a principle in detecting the residual polarization stored in the ferroelectric thin film 4500 based on difference in the value of current flowing through a ferroelectric transistor 4601 selected according to its address from among ferroelectric transistors as shown in FIG. 45 arranged in a matrix as shown in FIG. 46.
Also, there are various other types of ferroelectric memory. However, most of them use either a combination of ferroelectric capacitors and insulated-gate field effect transistors or field effect transistors with a ferroelectric thin film formed in the gate. Thus, they are regarded as basically similar kind and similar type in principle.
Incidentally, an example in which a ferroelectric capacitor 4349 or 4411 (such as shown FIG. 43 or FIG. 44) and insulated-gate field effect transistor 4412 are combined to be used as a memory element is disclosed in JP-A-11-39882. A similar example is disclosed in JP-A-11-177036 although it differs in the method for connecting the ferroelectric capacitor and insulated-gate field effect transistor.
Also, examples in which a field effect transistor 4601 with a ferroelectric thin film formed in the gate shown FIG. 45 or FIG. 46 is used as a memory element are disclosed in JP-A-11-251586 and JP-A-2004-153239.
However, in any of JP-A-11-39882, JP-A-11-177036, JP-A-11-251586, and JP-A-2004-153239, when using as the ferroelectric capacitor or field effect transistor with a ferroelectric thin film formed in the gate, elements must be made independent of each other. For that, the ferroelectric thin film must be separated element by element. Therefore, a technique has been adopted in which the ferroelectric thin film is cut chemically or physically or grown in small areas in isolation. Ferroelectric material varies greatly in characteristics at end points of crystals. Thus, in conventional configuration of ferroelectric memory, when a memory cell is miniaturized to increase the packing density of the device, the ferroelectric thin film must be reduced in size accordingly. However, the characteristics of the ferroelectric material may change with miniaturization as described above. As a result, there are problems that this makes it difficult to accomplish miniaturizing by means of miniaturization, and makes it difficult in turn to achieve high capacity and reduce costs.